1. Field of the Invention
The field of the invention is that of cross-linkable or cross-linked silicone compositions that can be used in particular to form a non-stick and water-repellent coating or film for flexible substrates, for example made of paper or the like, and in the form of natural or synthetic polymer films.
2. Description of Related Art
These hardenable non-stick silicone compositions are applied to such substrates so as to facilitate the removal of adhesive materials that are reversibly glued to these substrates.
These liquid silicone compositions are applied to the substrate films in industrial coating devices comprising cylinders operating at very high speed (for example 600m/min). It is clear that in these coating procedures at very high speed, the viscosity of the liquid silicone coating composition should be meticulously adjusted to the coating operating conditions.
In practice, the rate of deposition of non-stick silicone is between 0.1 and 2, preferably 0.3 and 1 g/m2, which corresponds to thicknesses of the order of a micrometer. Once applied to the flexible substrate, the silicone composition cross-links to form a solid water-repellent and/or non-stick silicone coating (for example elastomer).
Given the industrial rates of coating at very high speed, the kinetics of cross-linking should be extremely rapid to lead to proper cross-linking, that is to say that the non-stick silicone films should be sufficiently cross-linked in order to be able to fulfill as much as possible their role of anti-adhesion and have the desirable mechanical qualities. Assessment of the quality of cross-linking of the non-stick silicone coating may in particular be made through the assay of the non-cross-linked extractable compounds whose quantity should be as low as possible. For example, the amount of extractable materials is preferably less than 5%, under normal industrial cross-linking conditions.
The anti-adhesion of the free outer face of the silicone coating is expressed through the peeling force, which should be weak and controlled, for the element intended to be placed on the substrate coated by the non-stick silicone coating. Conventionally, this element may be the adhesive face of a label or of a tape with the same name.
Accordingly, in addition to this weak and controlled anti-adhesion, the adhesion of the silicone coating on its substrate should be very high. This property of adhesion is assessed, for example, using the rub-off trade test which consists in rubbing the surface of the coating with the finger and in measuring the number of successive passes before damage of the coating.
It is also important that these silicone coating compositions, which can be cross-linked by hydrosilylation, for example Si—H/Si-Vi, have the longest possible lifetime at room temperature when they are in the form of a coating bath in industrial coating machines.
The flexible substrates coated with a non-stick silicone film may be for example:                a paper or a polymer film of the polyolefin type (polyvinyl chloride (PVC), Polypropylene or Polyethylene) or of the polyester type (Polyethylene terephthalate or PET),        an adhesive tape whose inner face is coated with a layer of pressure-sensitive adhesive and whose outer face comprises the non-stick silicone coating;        or a polymer film for protecting the adhesive face of a pressure-sensitive self-adhesive or adhesive element.        
For obvious reasons of handling safety and toxicity, silicone compositions that are advantageously free of solvent are sought in the present invention.
Beyond this aspect, it is preferable, from an economic point of view, that these silicone coating compositions that are advantageously solvent free can be used on standard industrial coating machines suitable for flexible substrates made of paper. This assumes that said compositions have a relatively low viscosity (for example less than or equal to 1000 mPa·s) in order to facilitate their handling, in order to have a good coating quality and in order to reduce the problem of formation of mist (misting) which appears at very high industrial coating speeds.
Another constraint to be taken into account for the formulation of the liquid silicone coating compositions is that the coefficient of friction of the cross-linked silicone elastomer coating should be controllable, so as to facilitate the operations of rolling up/unrolling the flexible substrates made of polymer (in particular of polyester such as PET), which are useful as liners for labels.
It is important for this application that the elastomer silicone coating does not adversely affect the smooth appearance, or the transparency or the mechanical properties of the substrate. The smooth appearance and the mechanical properties are necessary for precision stripping at very high speed. The transparency is desirable for the high-speed inspection of the regularity of the film, using optical detectors.
In a non-stick coating, controlling the peeling force is important. Advantageously, this control should be effective at low and high speed. The equilibrium between the peeling forces at low speed and the peeling forces at high speed is commonly called the anti-adhesion profile.
Beyond the properties mentioned above, for all substrates it is first and foremost advisable that the adhesion or attachment of the silicone coating on the substrate (represented by the rub-off abrasion resistance) is optimum and stable over time, this being so even in the presence of the adhesive for the label.
Now, in the context of the invention, there is a focus more especially on the optimization of this parameter of adhesion or attachment of the silicone coating on the substrate, without detriment to the other specifications.
Non-stick silicone compositions are known from application EP159693-A2 which have improved adhesion on paper or polymer substrates and which comprise alkenylated linear polyorganosiloxanes MVi2-5 D50-1000 Ta′″≧0 M0-0.5, linear polyorganosiloxanes with hydrogenosiloxyl units (≡Si—H), a platinum catalyst, a cross-linking inhibitor, and, as attachment-promoting additive, one of the polyorganosiloxanes of the MDxM type bearing functional grafts of the epoxy, oxirane or carboxy type. More precisely, this additive may be of the M[DAGE]xD′M [AGE=allylglycidylether] or M[Dalkoxysilyl]x [Dcarboxy]xD′M, with D′=hydrogenosiloxyl unit (≡Si—H). This additive has the major disadvantage of not being able to be formulated with conventional cross-linking oils having hydrogenosiloxyl units. Indeed, when this type of additive is formulated with a cross-linking silicone oil having hydrogenosiloxyl units, phenomena of cloudiness and demixing appear in the formulation.